Here’s the UCMJ provision on voluntary manslaughter. The textbook example of VM in criminal law is someone walking in on their spouse in bed with someone else and shooting one or both of them in a blind rage. That “blindness” is what distinguishes the crime from murder, just as the intent involved distinguishes it from the lesser charge of negligent homicide (which prosecutors had been considering as an alternative). Presumably they’re going to argue that Wuterich simply went haywire after the IED went off that killed Lance Cpl. Terrazas and ended up gunning down the Iraqis as part of his rampage. The defense will probably try to show that Wuterich followed standard tactics and procedures in assaulting the houses, since the more evidence there is that he was thinking rationally and by the book, the harder it is to believe that he was out of his mind.
Even so, he’s looking at a whole lotta prison if convicted.